How can I rotate an UIImageView by 20 degrees?

A transformation matrix is not incredibly difficult. It's quite simple, if you use the supplied functions:

imgView.transform = CGAffineTransformMakeRotation(.34906585);

(.34906585 is 20 degrees in radians)

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Swift 5:

imgView.transform = CGAffineTransform(rotationAngle: .34906585)

If you want to turn right, the value must be greater than 0 if you want to rotate to the left indicates the value with the sign "-". For example -20.

CGFloat degrees = 20.0f; //the value in degrees
CGFloat radians = degrees * M_PI/180;
imageView.transform = CGAffineTransformMakeRotation(radians);

Swift 4:

let degrees: CGFloat = 20.0 //the value in degrees
let radians: CGFloat = degrees * (.pi / 180)
imageView.transform = CGAffineTransform(rotationAngle: radians)

Swift version:

let degrees:CGFloat = 20
myImageView.transform = CGAffineTransformMakeRotation(degrees * CGFloat(M_PI/180) )